1. Field of the Invention
The invention relates to the formation of catalytic adsorbents formed from the pyrolysis of different types of sludges alone or in combination with composting materials. The sludges include municipal, industrial, waste oil and metal based sludges. The composting materials can include tobacco waste.
2. Discussion of the Related Art
Growing concerns about the environment has resulted in the development of new environmentally friendly technologies, new materials, and new ways to reduce and minimize wastes. One of the wastes produced by contemporary society in abundant quantity is municipal sewage sludge, often referred to as biosolids. Biosolids are a mixture of exhausted biomass generated in the aerobic and anaerobic digestion of the organic constituents of municipal sewage along with inorganic materials such as sand and metal oxides. Other sludges include wastes from such industry as shipyards, foundry, or paper mills. It is estimated that about 10 million dry tons of sewage sludge is produced in the United States. Moreover, Sweden alone contributes 220,000 dry tons of sludge to the 8-10 million tons of dry sludge produced by European Union.
Various methods have been used to dispose of or utilize municipal sewage sludge, including incineration, landfilling, road surfacing, conversion to fertilizer, compression into building blocks, and carbonization. Since 1976, several patents have been issued on carbonization of sewage sludge and various applications of the final materials. Carbonization of sludge in the presence of chemical activating agents such as zinc chloride and sulfuric acid produces new sorbents, with patented applications in processes such as removal of organics in the final stages of water cleaning and removal of chlorinated organics. Industrial sludges after dewatering processes/drying are ether used as landfills or disposed mainly as hazardous wastes.
Carbonization of sludge to remove pollutants either from gas of liquid phase, is based on the fact that typically activated carbons are chosen. This is owing to their large surface area and high volume of pores. Often, these characteristics of activated carbons are not potent enough to retain certain molecules, especially small ones, for which the dispersive interactions with the carbon surface are rather weak. In such cases, the carbon surface has to be modified to impose the specific interactions. These interactions include hydrogen bonding, complexation, acid/base reactions or redox processes. Fortunately, in the case of carbons, various technologies leading to modified surfaces exist and are relatively easy to achieve. Examples are oxidations with various oxidants such as strong acids, ozone, or air, impregnation with catalytic metals or reducing/oxidizing compounds, heat treatment in the presence of heteroatom sources such as chlorine or nitrogen compounds, and others.
As a result of the treatments mentioned above, new functional groups/chemical species are introduced to the surface. They impose the specific and/or chemical interactions with the species to be removed. To have the removal process efficient, the chemical state of these species and their dispersion on the surface are important issues. Another important challenge is preservation of carbon porosity which is a crucial asset for the retention/storage of pollutants. Thus, the surface modifications can be done in such a way in which a minimal decrease in the surface area/pore occur.
Taking into account the above requirements, in some cases modifications of a carbon surface, besides being a challenge, can also be associated with high expenses, especially when noble or catalytic metals are involved. Industrial sludges, as those coming from shipyards or other heavy metal industries, are rich in catalytic transition metals. By pyrolysis of these materials, not only the volume of waste is reduced but those environmentally detrimental wastes can be recycled and converted into valuable products. These products, when used, can be safely disposed since the leaching of materials is significantly reduced by mineralization of those metals via high temperature solid state reactions.
The process of carbonization of sewage sludges has been studied in detail previously and it is described in the literature. Materials obtained as a result of the treatment have surface areas between 100 and 500 m2/g. Their performance as adsorbents of hydrogen sulfides, sulfur dioxide, basic or acidic dyes, phenol or mercury has been reported as comparable or better that that of activated carbons. In many process the excellent sorption ability of these materials is linked to the catalytic action of metals present in various forms in the final products. Their chemical forms along with the location on the surface were reported as important factors governing the pollutant removal capacities. In some case the wastes were mingled and, owing to the synergy between the components, more efficient adsorbents were obtained.
Adsorbents obtained by pyrolysis of sludge can be considered as complex pseudocomposite materials. However, the process of carbonization of biosolids has been studied in detail previously and it is described in the literature. It has been recently shown that by simple pyrolysis of municipal sewage sludge derived fertilizer, Terrene®, exceptionally good adsorbents for removal of sulfur containing gases can be obtained. The removal capacity is twice that of coconut shell based activated carbon. Although, it was attributed to the specific combination of inorganic oxides of such metals as iron, copper, zinc or calcium. The predominant influence of inorganic phase or combination of oxides, which are also quite commonly used as catalysts for hydrogen sulfide oxidation or sulfur dioxide adsorption, was ruled out on the bases of the performance of a pure inorganic phase in the removal of sulfur containing gases. The capacity of pure inorganic phase heated at 950° C. was negligible. The data also showed that the oxidation of hydrogen sulfide occurs until all micropores (mainly about 6 Å in size), likely within carbonaceous deposit or on the carbon/oxide interface, are filled with the reaction products. The form of that carbonaceous deposit is important and that deposit may play a role in adsorption capacity.
The products of oxidation immobilized on the surface are stored there. Table 1 shows the capacity of sewage sludge derived materials as adsorbents of sulfur containing gases. For removal of a toxic gas containing reduced sulfur the capacity is much greater than that of activated carbons. It happens in spite of the fact that the carbon content is small (about 20%) and pore volume much smaller than that of carbons.
TABLE 1H2S and SO2 breakthrough capacities for sludge derived adsorbents(SC series) and activated carbon (S208). The number after SCrefers to the temperature of heat treatment in Centigrade.H2S breakthroughSO2 breakthroughSamplecapacity [mg/g]capacity [mg/g]SC-4008.25.1SC-60014.99.5SC-80023.622.2SC-95082.629.8S20848.848.2
Since pore volume seems to be a limiting factor for the capacity of sewage sludge derived materials, an increase in the content of carbonaceous deposit and pore volume with maintaining the desired content of a catalytically active phase seems to be the desired direction of feature research. Resent studies showed that the pore volume active in the removal of such compounds as hydrogen sulfide does not need to be in pores similar in size to adsorbent molecule. Since the catalytic oxidation is the predominant mechanism of adsorption, the larger pores, (meso- and macropores) where the product of oxidation is stored were found to be beneficial.
Another important factor is the chemistry of a catalytic phase, its dispersion, location on the surface, compatibility with the carbon phase and the effects of both phases on the removal process (adsorption/catalytic oxidation/storage). It was found that excellent capacity of an expensive desulfurization catalyst, US Filter carbon Midas®, is linked to the presence of calcium and magnesium oxides dispersed within the microporous activated carbon. On this catalyst, hydrogen sulfide is oxidized on basic centers of alkali earth metal oxides and sulfur is formed. The fact that this carbon is able retain up to 60 wt % sulfur is linked to a limited reactivity of MgO and CaO. On their surface, due to the basic pH and the presence of moisture, sulfur is formed and owing to the close proximity of the carbon phase, that sulfur migrates to the high-energy adsorption centers, small pores. In this way the catalytic centers are renewed and the adsorbents works until all small pores are filled by sulfur.
Sewage sludge based materials were also found as efficient adsorbents for removal of mercury from waste water and copper. Other common industrial pollutants which can be efficiently removed using those materials are basic and acidic dyes. In the case of these adsorbates the high capacity is linked to surface chemical nature (acidic and basic sites) and relatively large pores which are similar in size to the molecules of organic dyes.
At high temperature, the organic matter vaporizes, dehydrogenation occurs and carbon can be deposited back on the surface of an inorganic support as carbon nanotubes of filaments. This may happen due to the presence of highly dispersed catalytically active metals. Since this process resembles the chemical vapor deposition (CVD), it is referred to as the self-imposed chemical vapor deposition (SICVD). The process of carbon nanotube growth on the catalysts containing nickel or cobalt is well-known and described in the literature. The nanotubes and carbon filaments grow on metal “seeds” and their effective size depends on the sizes of the seeds. Introduction of more carbon phase can increase the porosity leading to more space for storing of oxidation products and also can lead to the formation of greater quantity of novel carbon entities in the process of CVD. FIG. 1 shows an SEM image of carbon nanotubes grown on the surface of sewage sludge-derived materials.
The carbon and nitrogen content of the sludge plays a role in the formation and properties of the adsorbent. While municipal sewage sludge is a promising material to use as a base with other waste sludges, other carbon or nitrogen based wastes can also be used. Besides formation of new carbon entities in the presence of catalytic metals as a result of heat treatment the new spinel-like/mineral like active components can be formed. Recently, for some sewage sludges containing iron and calcium the catalytically important entities were identified as dicalcium ferrite (Ca2Fe2O5).